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uSummary
uFigure
captions
uStructure
uTectono-stratigraphy
u Stratigraphic
implications
uAcknowledgments
uReferences
uSummary
uFigure
captions
uStructure
uTectono-stratigraphy
uStratigraphic
implications
uAcknowledgments
uReferences
uSummary
uFigure
captions
uStructure
uTectono-stratigraphy
uStratigraphic
implications
uAcknowledgments
uReferences
uSummary
uFigure
captions
uStructure
uTectono-stratigraphy
uStratigraphic
implications
uAcknowledgments
uReferences
uSummary
uFigure
captions
uStructure
uTectono-stratigraphy
uStratigraphic
implications
uAcknowledgments
uReferences
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Figure Captions
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Figure 1. Geodynamic setting of Eastern
Venezuela Basin (EVB). Present day plate structure of the
Caribbean region. Subduction type B to the east between Atlantic
and Caribbean-South American plates. At the north, subduction
type B of the Caribbean Plate. In the south, Subduction type A
of the South American plate. (Modified from Jacome 2001, after
DeMets et al. 1994, Kellogg, et al. 1995, Mascle and Letouze,
1990, and Audemard and Lugo, 1996.) Topographic and bathymetric
map are from The National Geophysical Data Base, 1988. |
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Figure 2. Seismic data base--1000 km 2-D
and 700 km2 3D. |
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Figure 3. Surface geology base
map. |
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Figure 4. Map of tectonic provinces. The
final results provided by this study, enabled the separation of
the Pirital block in two different sub-blocks: Pirital-Cerro
Corazon block and Manresa block (see section C-C’). (Map
modified after Cobos, 2002, and Chaplet, 2001.) |
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Figure 5.
Strike section showing the different tectonic sheets
laterally. Note that the Pirital-Tala thrust systems (system in
blue) are composed of various thrust families separated by
lateral ramps. This explains the differences between dip
sections that lie in different tectonic sheets. This new
interpretation of the Pirital block enabled us to develop the
proposition of a new lateral ramp within the Pirital block,
delimiting two laterally different tectonic provinces: the
Pirital-Cerro Corazon block and the Manresa block. |
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Figure 6. NW-SE dip seismic cross
section showing the principal seismic termination and
interpretation of four main unconformities. |
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Figure 7.
Surface structural cross section across the Interior Ranges,
showing the structural styles of the outcropping ranges. The
thrusts painted in red correspond to the youngest-short
wavelength thrust system. |
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Figure 8.
Same cross-section as Figure 5.
Three main thrust systems (red, blue,
and green) with different basal detachment, emplaced at
different periods of time forming structures with different
wavelength. The oldest system in red is interpreted to be
bisected (in two), after the first emplacement of the Pirital
thrust system (painted in blue), burying some of the structures
with depth to the south (Monagas giant oil field) and exposing
the others in the outcropping ranges. The oldest system (painted
in green) would reactivate the main Pirital thrust, uplifting
and rotating the Pirital high and forming the Morichito Basin. |
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Figure 9. Spatial correlation of the
stratigraphic sequences, the structural model, and the
biostratigraphic data, based on a strike section (Figure
5). The integration of the
paleo-environmental curves was obtained through the compendium
of the biostratigraphic data available for the respective wells.
Note that the sequence M2 shows dramatic changes in
paleo-environments from South to North and also from West to
East. |
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Structural Interpretation
Three main tectonic provinces define the
Monagas foreland thrust belt: The Interior Ranges -“Serrania del
Interior,” the Pirital block, and the Monagas foothills.
The integration of subsurface,
seismic-structural interpretation, and surface structural profiles
enabled the description and characterization of the structural styles
for each of these three main tectonic provinces. Three main thrust
systems were interpreted to have been emplaced at different periods. The
youngest thrusts (highlighted in red color) generated smaller
short-wavelength anticlines. The oldest thrust systems (assigned in blue
and green colors) generated wider structures reactivating and deforming
previous thrusts.
Three-dimensional correlation of regional
seismic profiles tied to surface features shows that not only three
different thrust systems can be identified in cross sections, but also
two different families of thrusts can be correlated in a map view. These
two different sets of thrusts are bounded by their respective lateral
ramps. The systems are named here as the First Pirital thrust system to
the west, and the Second Pirital thrust system to the east (Figure
4). The southeastern end of the very well known “Urica fault zone”
has been interpreted as the western lateral ramps of the First Pirital
system. The subsurface interpretation of a new system of lateral ramps (Figure
5) to the east of Urica and to the south of the also known “San
Francisco fault” was the principal criteria to divide the Pirital system
into two separate families of thrusts. As a result, it is proposed here
that the Pirital block should be divided into two different blocks: the
Manresa block and the Pirital-Cerro Corazon block (Figure
4).
Tectono-Stratigraphic Interpretation
Seismic stratigraphy and biostratigraphic data
allowed the documentation of three main unconformities (Figure
6), each one of which dates the emplacement of the three thrust
systems interpreted in this study.
The oldest unconformity, dated early Miocene,
documents the emplacement of the first tectonic pulse recognized. High
frequency-short wavelength asymmetric anticlines characterize the
structural style for this period of deformation. Currently, most of
these structures are exposed to the north, in the outcropping ranges (Figure
7). To the south, in the subsurface, they are buried below a thick
column of foreland sediments and form the giant oil fields typical of
the Northern Monagas foredeep.
Middle Miocene sediments onlapping erosional
truncations (Figure 6) provide evidence and
timing for a second tectonic event. The thrust system (Pirital thrust
system), associated with this event, intercepted, uplifted, folded, and
reactivated the previous thrusts, exposing some of the oldest faults in
outcrops to the north and burying the rest of them in subsurface to the
south. The basal detachment of the Pirital thrust system has been
interpreted to lie within pre-Cretaceous rocks. Thus, more than 5 km of
Cretaceous and pre-Cretaceous strata have been folded, uplifted, and
transported to the south for more than 40 km of average displacement.
Upper Miocene strata onlapping middle Miocene
sediments give evidence for the emplacement of a third thrust system,
with its basal detachment interpreted to lie at the top of the basement
(Figure 8, system in green). Major thrusts
reactivated and deformed the Pirital structure, causing the rotation and
uplifting of the Pirital high and the creation of the Morichito basin.
It is also proposed in this study that some of the major faults within
the Monagas basin, such as the Urica fault and the San Francisco fault,
constitute the lateral ramps of this last major event.
Stratigraphic
Implications
Structural-stratigraphic integration (Figure
9) allowed the spatial correlation of the tectono-stratigraphic
sequences. The regional strike section (Figure 5)
was the key to this integration. The lack of lateral correlation within
some stratigraphic units was better understood when realizing that the
Pirital block is composed of more than one thrust systems.
The tectono-stratigraphic evolution of the
Pirital block, according to the model presented in this study, enables a
better understanding of the Morochito basin history. Traditionally, this
basin had been interpreted as a piggy back basin, formed after the
emplacement of the Pirital thrust. However, the interpretation of two
different unconformities within this basin, the evidence of a deeper and
younger thrust system, reactivating and deforming the previous Pirital
thrusts, together with the interpretation of seismic-stratigraphic
relations tied to biostratigraphic data, indicate that the Morichito
basin has more than one episode of formation. As opposed to traditional
interpretations, here two different tectono-stratigraphic sequences are
defined within the Morichito basin, named sequences M2 and M3, middle
Miocene and late Miocene, respectively (Figure 9).
I want to thank Jose Humberto Sanchez and his
exploration team from PDVSA, who proposed the project, tutored and
followed the process to its end. This work was possible thanks to the
kind orientation and advice of my mentors from PDVSA Exploration, Raul
Ysaccis and Felipe Audemard. Special thanks to my academic mentor
Franklin Yoris from Simón Bolívar University, for his support and
guidance.
Audemard, F.E., and Lugo, J.,
1996, Petroleum geology of Venezuela: AAPG Annual Meeting, Caracas,
Venezuela, 1996, Short Course.
Duerto, L., and McClay, K.,
2002, 3D geometry of shale diapirs in the Eastern Venezuela Basin:
Search and Discovery Article # 10026 (2002). Adapted for online
presentation from poster session by the authors at the AAPG Convention,
Houston, Texas, March, 2002.
Jacome, M.I., 2001, The
formation of the Monagas foreland basin: Eastern Venezuela: Ph.D.
Thesis, University of Liverpool, Liverpool, England, U.K., 204 p.
National Geophysical Data Center, 1988, ETOPO-5, bathymetry/topography
data. Data Announcement 88-MGG-02: National Oceanic Atmospheric
Administration, US Department of Commerce, Washington, DC.
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